Preheat oven to 350°F (175°C). Line a sheet pan with parchment paper.
Slice ciabatta loaf in half. Place bottom on pan. Place top cut-side up and brush with olive oil. Sprinkle with Italian seasoning.
Spread Dijon mustard and mayo on bottom half. Layer with ham, pepperoni, mozzarella, salami, cappicola, and another layer of cheese.
Bake 6–8 minutes until cheese melts and bread is toasted.
While baking, mix lettuce, pepperoncini, juice, onion, vinegar, Dijon, garlic powder, salt, and pepper.
Remove sandwich from oven. Add lettuce mixture and tomato slices. Spread mayo on top half of bread, then assemble.
Slice and serve hot.